X-inactivationX-inactivation

Males are haploid for X-linked Males are haploid for X-linked genesgenes

Barr body discovered (1949)Barr body discovered (1949)

Lost twin?

Lyonization=X-inactivationLyonization=X-inactivationThe hypothesis:• The heteropyknotic

(..”condensed”) X chromosome was inactive.

• Paternal or maternal in origin.

• The inactivation occurred early in embryonic development.

The genetic facts that helped formulate the hypothesis:

• X0 mice are normal fertile females (One X is enough).

• Female mice heterozygous for coat colour show mosaic phenotype.

Dosage compensationDosage compensation

• Dosage compensation refers to a regulatory mechanism that ensures the equalization of X-linked gene products in males and females.

Random X-inactivation (Placental Random X-inactivation (Placental mammals)mammals)

Random Vs. Imprinted X-inactivationRandom Vs. Imprinted X-inactivation• Imprinted refers to the

skewed inactivation of the paternal X-chromosome.

• Imprinted X-inactivation is found among marsupials and also in the extra-embryonic tissues in a subset of placental mammals.

)Duret et al., 2006(

Xic (X-inactivation center) locusXic (X-inactivation center) locus

Ectopic expression of Xic transgene leads to X-inactivation in male cells .

Derivation of embryonic stem (ES) Derivation of embryonic stem (ES) cells from blastocystscells from blastocysts::

Mouse embryonic stem cells derived and cultured in-vitro- 1981 (Martin et al)

Human embryonic stem cells derived and cultured in-vitro-1998 (Thomson et al)

ES cell give rise to the ~220 cell ES cell give rise to the ~220 cell types that comprise the human types that comprise the human

bodybody

I. Self renewal- Unlimited division while maintaining a pluripotent undifferentiated state in-vitro.

II. Differentiation- The ability to differentiate to cells from the three embryonic germ layers (in-vivo and in-vitro).

Neo- positive selection for integration.

HSV-TK- negative selection for recombination that are not homologous

Targeted deletion of Xist in ES cells (1996)

Xist allele of 129 (From 129/PGK) was deleted using Homologous Recombination.

Mutant clone – The probe does not recognize 129 allele but does recognize PGK allele (“B”)- the 129 Xist allele was deleted (“A).

Targeted deletion of Xist in ES cells Targeted deletion of Xist in ES cells (1996)(1996)

• Option I: The mutant ES cells will fail to undergo X inactivation.

• Option II: The X-chromosome bearing the Xist mutation (129) will fail to x inactivate.

• Option III: The mutation will have no effect at all on X-inactivation.

X-related gene expression detected only in B -> no inactivation of the Xist deleted X chromosome (129 is B). This leads to skewed monoallelic

expression between clones because A (PGK) is always inactivated! In activation is in CIS

Xist RNA coats the inactive X Xist RNA coats the inactive X (FISH )(FISH )

Chromatin modification during initiation of X-chromosome inactivation

And Loss of euchormatin-associated histone modifications (H4 acetylation)

And DNA methylation

Xist spreading is LINE dependent

• The x-chromosome is enriched for LINE sequences compared to autosomes.

•These may serve as “way station” for Xist binding (due to high affinity for RNA binding).

• Tanslocation of X/4 chromosomes showed that Xist spreading ended at the translocation point (correlates with a drop in LINEs sequences in chromosome 4).

Xic pairing in ES undergoing Xic pairing in ES undergoing differentiation (Bacher 2006)differentiation (Bacher 2006)

• The pairing was observed only in ES undergoing differentiation.

•In Xic-Del ES cell line no pairing was observed.

•The X chromosome harboring the deletion was repeatedly the one undergoing inactivation.

TsixTsix

• A long untranslated RNA transcribed in the anti-sense direction of Xist which represses Xist by forming dsRNA hybrid that is processed and degraded.

• Initially expressed on both X chromosomes (In ES) but persists on the active X after differentiation. It is downregulated from the Inactive X.

• Tsix is regulated by DXpas34 and Xite elements- Deletion of DXpas34 leads to X inactivation.

• Disruption of Tsix induces X-inactivation in XY ES.

?

Unresolved QuestionsUnresolved Questions

• What are the mechanisms for choosing and counting?

• How does the spreading along the chromosome occur?

• How does X inactivation maintained in the female soma?

• What is the difference between imprinted and random X inactivation?

• How is X inactivation coupled with cell differentiation?

20

Nanog Oc4 and Sox2 are the master regulators Nanog Oc4 and Sox2 are the master regulators of the pluripotent undifferentiated state of ES of the pluripotent undifferentiated state of ES

cellscells

Oct4 Sox2 Nanog

Undifferentiated pluripotent ES colony (very small round cells) cultured in-vitro.

ES nucleus is positively stained (pink) for Nanog

Feeder cell nucleus are not stained for Nanog (blue)

Molecular Coupling of Xist and Molecular Coupling of Xist and pluripotency (Navarro 2008)pluripotency (Navarro 2008)

Oct4, Sox2 and Nanog transcription factors (TFs), ,which are the master regulators of the pluripotent state in ES, bind Intron1 of Xist thus repressing its

transcription in ES cells .

When ES are induced to differentiate, Oct4, Sox2 and Nanog are downregulated and Xist is freed from their binding “grip” and rapidly accumulates

on one of the X-chromosomes .

Binding site of Nanog in intron1 of Xist

WT ES=>Xist is repressed. -/-Nanog ES=> Xist is upregulated.

OCT4 controls pairing and countingOCT4 controls pairing and counting

• Control XCI.

• Triggers pairing and counting.

•Binds Xite and Tsix.

•Depletion- blocks pairing and induces XCI in both Xs.

• If normal XX female has one X inactivated, why is a X Turner female not normal?

• Similarly, if XXY male has one X inactivated, why does he have Klinefelter syndrome?

Inconsistencies between syndromes and X inactivation

Escape from X-inactivation ?

24

Carrel et al (2005) showed that ~15% of the genes on the inactive-X escaped from inactivation, complicating the story even further..